There are actually quite a few reasons why the steering wheel isn't located in the center of the dashboard.
Seating for Adults
A car would have to be much wider to comfortably seat three adults in the front seating row. Two adults in the front seat would cause the driver to sit slightly to the left or right. Widening the car would make it more difficult to park, and increase wind resistance.
The steering column needs room to connect to the other steering components under the hood, and in most cars (the ones with their engine in the front, specifically), the engine would be in the way, and routing the steering linkage around the engine would make the steering heavy.
While sitting in the middle would mean both sides of the car are equally distant from the driver, it would also make having a center mirror impractical, and it would move the driver further from the center of the road, making it hard to see around some corners, such as those at intersections/crossroads.
A seat in the middle of the vehicle would take more effort to get in and out of easily, which is why many race cars still have left or right side seating.
Some special exceptions exist that break the norm of steering wheel location. For example, the McLaren F1 seen from inside at the right has a racing-oriented design and focus, leading to a center-mounted driver's seat with rear-view mirrors on both sides of the front window, instead of the typical single center-mounted mirror.